TITLE: GCN GRB OBSERVATION REPORT
NUMBER: 1618
SUBJECT: GRB 021004: WC wind?
DATE: 02/10/10 23:15:39 GMT
FROM: Jules Halpern at Columbia U.
N. Mirabal, J. P. Halpern (Columbia), R. Chornock, and A. V. Filippenko
(UC Berkeley) report:
"Continued monitoring of the optical afterglow of GRB 021004 with the
MDM 1.3m by E. K. Armstrong yields the following measurements:
----------------------------
UT R +/-
----------------------------
Oct. 8 07:05 20.48 0.03
Oct. 9 04:21 20.72 0.04
----------------------------
Here we use the calibration of Henden (GCN #1583) in which the reference star
has R=15.52. An updated decay curve accounting for revised calibrations of
GCN reported data is at:
http://www.astro.columbia.edu/~jules/021004.ps
Further examination of the optical spectrum reported by Chornock and Filippenko
(GCN #1605) yields the following features in the highest redshift system
("em" refers to emission, "bl" denotes a broad blend of z=2.323 and 2.318):
-------------------------------------------
Line Rest wavelength z
-------------------------------------------
Ly beta 1025.7 2.323 bl
2.291
Ly alpha 1215.7 2.328 em
2.319 bl
2.295
Si IV 1393.8,1402.8 2.323
2.318
2.293
C IV 1548.2,1550.8 2.323
2.317
2.293
Al II 1670.8 2.328
-------------------------------------------
This spectrum was taken in five 1200 s exposures from Oct 8 10:14-14:05 UT.
In view of the 6 Angstrom resolution of this spectrum, we consider that the
lowest redshift absorber of the three is a blend of two of the four systems that
were seen by Salamanca et al. (GCN #1611) in higher resolution data. Also, we
detect one absorption line (Al II 1670) at the same redshift as the Ly alpha
emission, z=2.328, which we therefore designate the redshift of the host galaxy.
The detection of this lone low-ionization absorber is plausible because of its large
oscillator strength, f=1.83. Thus, all of the high-ionization absorbers are
blueshifted from the host redshift, by up to 3,200 km/s.
The decay curve shows significant deviations from a power law, which might be
understood in terms of inhomogeneities in the ambient medium in which the afterglow
expands. In view of the revised redshift of 2.328 and the multiple blueshifted
H, C IV, and Si IV absorption lines, we hypothesize that the velocity span of
3,200 km/s in these components is indicative of a clumpy wind from a massive hot star
progenitor, such as a Wolf-Rayet star. This would avoid the difficulties of scenarios
involving supernova ejecta or cosmic clustering, as explained in GCN #1611. The absence
of nitrogen lines and the presence of Ly alpha and Si IV weaker than C IV is consistent
with a WC star, in which the bulk of the wind is approximately equal parts He and C.
We assume a typical mass-loss rate of 10(-5) Msun/yr and a wind terminal velocity
of 3,000 km/s. Following Chevalier & Li (2000, ApJ, 536, 195), the predicted radius
r_s of a spherical adiabatic blast wave in such a stellar wind at an observed time of
1 day is 3.5x10(17) cm for an isotropic energy of 5.6x10(52) erg (Malesani et al.,
GCN #1607), or 3.5x10(16) cm if there is a beaming correction of 1/100 to the energy.
Chosing the larger value of r_s, the column density of C in the wind is at least
3.6x10(15) cm(-2) between the shock and the deceleration radius of the wind, which
is of order 5x10(17) cm or greater (Ramirez-Ruiz et al. 2000, MNRAS, 327, 829). This
would be enough to account for the observed absorption at high velocities. We note that
for a typical nu(-1) broad-band spectrum, C IV ions in the wind would absorb in the line
before being photoionized. Detailed photoionization models are needed to predict the
evolution of the column densities. A consequence of this model is that the high-velocity
components of C IV should decline after being photoionized, or as the shock overruns the
wind in several days. Continued spectroscopic monitoring of this bright afterglow is
urged in order to search for this definitive signature of the massive star progenitor of
GRBs. Changes in the decay curve could also result when the afterglow reaches the
termination of the stellar wind with the ISM."